Trying to restate the proposal, somewhat more formal following Random832
and Paul's suggestion.

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*The suggested change of syntax:*
comprehension ::= starred_expression comp_for
*Semantics:*
(In the following, f(x) must always evaluate to an iterable)
1. List comprehension:
result = [*f(x) for x in iterable if cond]
Translates to
result = []
for x in iterable:
if cond:
result.extend(f(x))
2. Set comprehension:
result = {*f(x) for x in iterable if cond}
Translates to
result = set()
for x in iterable:
if cond:
result.update(f(x))